As a new generation X-ray diagnostic detector, a planar X-ray detector based on an active matrix has been developed. By detecting X rays applied to this X-ray detector, an X-ray radiograph or a real-time X-ray image is outputted as a digital signal. In this X-ray detector, X rays are converted to visible light, i.e., fluorescence, by a scintillator layer. This fluorescence is converted to a signal charge by a photoelectric conversion element such as an amorphous silicon (a-Si) photodiode and CCD (charge coupled device) to obtain an image.
As the material of the scintillator layer, various materials are generally known, such as sodium-doped cesium iodide (CsI:Na), thallium-doped cesium iodide (CsI:Tl), sodium iodide (NaI), and gadolinium oxysulfide (Gd2O2S). Different materials are used depending on the purpose and required characteristics.
In the scintillator layer, grooves may be formed by dicing and the like, or a columnar structure may be deposited by evaporation technique. Thus, the resolution characteristics can be improved.
Above the scintillator layer, a reflective film may be formed for the purpose of enhancing fluorescence utilization efficiency to improve sensitivity characteristics. The reflective film is configured to increase fluorescence reaching the photoelectric conversion element side by reflecting the fluorescence emitted by the scintillator layer and directed opposite to the photoelectric conversion element side.
In an example known as a method for forming a reflective film, a metal layer having high fluorescent reflectance made of e.g. silver alloy or aluminum can be formed on the scintillator layer. In another example, a reflective film with light scattering reflectivity made of a light scattering material such as TiO2 and a binder resin can be formed by coating. As an alternative method in practical use, instead of formation on the scintillator film, a reflective plate having a metal surface of e.g. aluminum can be brought into close contact with the scintillator layer to reflect fluorescence.
Furthermore, a moisture-proof structure is used for the purpose of protecting the scintillator layer and the reflective layer (or reflective plate and the like) from the external atmosphere to suppress characteristics degradation due to moisture and the like. The moisture-proof structure is an important component in providing the detector as a practical product. In particular, the CsI:Tl film and CsI:Na film are materials highly prone to degradation by moisture. In the case of using these films for the scintillator layer, high moisture-proof performance is required.
In a conventional moisture-proof structure, a hat-shaped moisture-proof body made of e.g. Al foil is adhesively sealed with the substrate in the peripheral portion to maintain the moisture-proof performance.
In this structure, the Al hat-shaped moisture-proof body is adhesively sealed with the substrate in the flange portion of the moisture-proof body. This structure is obviously superior in moisture-proof performance to that based on a moisture-proof body of an organic film made of e.g. polyparaxylene. Furthermore, most of the array substrate is covered with a conductive material. This also achieves a noise reduction effect by the shielding effect.
In the aforementioned radiation detector, pixels are formed in a matrix on the substrate. The pixel includes a photoelectric conversion element such as photodiode, and a switching element (TFT). The substrate is divided into an active area, a bonding area, a TAB pad section and the like. The active area is a pixel region on which the scintillator layer is formed. In the bonding area, the moisture-proof body covering the scintillator layer is bonded to the substrate via an adhesive layer. In the TAB pad section, the wiring from the active area is electrically connected to the circuit external to the substrate.
On the active area, a scintillator layer such as CsI:Tl film is formed by vacuum evaporation technique and the like. Here, the adhesion strength between the scintillator layer and the substrate is very important for the subsequent process and the reliability of the product after shipment. In the case where the adhesion strength is weak, the scintillator layer may be peeled from the substrate. This causes characteristics degradation and in-plane characteristics variation, being fatal to the product. In particular, a reflective film made of TiO2 fine grains and binder resin may be formed by coating and drying on the scintillator layer. In this case, the scintillator layer is easily peeled from the substrate by the stress at the time of drying the reflective film and the stress due to thermal expansion difference between the states of the product at high temperature and low temperature. Thus, in view of ensuring this adhesion strength, the material of the substrate outermost layer to which the scintillator layer is attached is important.
On the other hand, in the bonding area, the material of the adhesive layer and the adhesion strength to the substrate are important. Naturally, this adhesion strength needs to be ensured at the initial state. Furthermore, long-term moisture-proof reliability also needs to be ensured by suppressing degradation in the high-temperature high-humidity state and cold-hot state. The degradation of adhesion strength leads to the degradation of moisture-proof sealing performance, and results in moisture permeation through the interface between the moisture-proof body and the adhesive layer, through the interface between the adhesive layer and the substrate, or from the adhesive layer itself. This causes characteristics degradation of the internal scintillator layer such as CsI:Tl film.
In general, the outermost layer material of the substrate suitable for ensuring the adhesion strength to the adhesive layer does not coincide with the outermost layer material of the substrate suitable for ensuring the adhesion strength to the scintillator layer such as CsI:Tl evaporated film. Thus, the conventional material used for the substrate outer layer is not insufficient for one or both of the adhesion strength to the scintillator layer and the adhesion strength to the moisture-proof body. This often leads to poor reliability in the cold-hot environment and high-temperature high-humidity environment.